The present invention relates to a double-side reading system for reproducing a radiation image which is stored in a radiation image storage panel containing stimulable phosphor particles.
As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed, and has been practically employed. The method employs a radiation image storage panel (i.e., stimulable phosphor sheet) comprising a stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as xe2x80x9cstimulating raysxe2x80x9d) to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The panel thus treated is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
In the above-mentioned radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with a conventional radiography using a combination of a radiographic film and radiographic intensifying screens.
The radiation image storage panel has a basic structure comprising a support and a stimulable phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. On the free surface (surface not facing the support) of the phosphor layer, a protective film is generally placed to keep the phosphor layer from chemical deterioration or physical shock.
The stimulable phosphor layer usually comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor particles without binder. The phosphor layer containing no binder can be formed by deposition process or sintering process. Further, a phosphor layer comprising agglomerated phosphor particles which are soaked with a polymer is also known. In any type of the phosphor layers, the stimulable phosphor particles emit stimulated emission when excited with stimulating rays after having been exposed to a radiation such as X-ray. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the dose of the applied radiation, and a radiation image of the object is produced and stored in the storage panel in the form of a radiation energy image. The radiation energy image can be released as stimulated emission by sequentially irradiating the storage panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recorded in the storage panel is generally read by the steps of applying the stimulating rays onto the front surface side (phosphor layer side) of the storage panel, collecting light (i.e., stimulated emission) emitted by the phosphor particles by means of a light-collecting means from the same side, and photoelectrically converting the light into digital signals of image. A system for reading the image from one side of the storage panel in this manner is referred to as xe2x80x9csingle-side reading systemxe2x80x9d. However, there is a case that the light emitted by the phosphor particles should be collected from both sides (i.e., front and back surface sides) of the storage panel. For instance, there is a case that the emitted light should be collected as much as possible. There is also a case that the radiation image recorded in the phosphor layer varies along the depth of the phosphor layer, and that the variation should be detected. A system for reading the image from both sides of the radiation image storage panel is referred to as xe2x80x9cdouble-side reading systemxe2x80x9d. In Japanese Patent Provisional Publication No. 55 (1980)-87970, for example, the double-side reading system is described.
A radiation image storage panel employed in the double-side reading system, as well as a panel employed in the single-side reading system, is desired to be as sensitive as possible and to provide an image of high quality (high sharpness, high graininess, etc.).
In order to improve the quality of an image obtained by the conventional single-side reading system, various radiation image storage panels were proposed. For example, Japanese Patent Provisional Publication No. 59-139000 discloses a radiation image storage panel having plural phosphor layers each of which contains stimulable phosphor particles of a different mean particle size. Further, Japanese Patent Provisional Publication No. 59-138999 discloses a radiation image storage panel having a phosphor layer containing stimulable phosphor particles whose particle size distribution has two or more peaks.
The double-side reading system generally gives an image with high sensitivity, but from the viewpoint of image quality such as sharpness and graininess it is not always satisfactory. If an image recorded in a known radiation image storage panel is simply read by the double-side reading system in accordance with conventional knowledge and techniques for the single-side reading system, the radiation image obtained from the back surface side often exhibits poor quality (particularly, poor graininess at a high spatial frequency). Accordingly, the visible image reproduced by combining the signals thus obtained from both sides has poor quality.
It is an object of the present invention to provide a double-side reading system utilizing a radiation image storage panel by which an radiation image of improved quality (particularly, improved graininess at a high spatial frequency) can be obtained also from the back surface side.
The present invention resides a process for recording and reproducing a radiation image comprising the steps of:
applying a radiation having passed through an object or having radiated from an object onto a radiation image storage panel to absorb energy of the radiation, said radiation image storage panel comprising stimulable phosphor particles wherein the stimulable phosphor particles vary in their mean particle sizes in the depth direction of the storage panel in such manner that the mean particle size on one surface side is smaller than the mean particle size on the other surface side;
sequentially applying stimulating rays onto the radiation image storage panel, preferably on the surface side having the stimulable phosphor particles of the smaller mean particle size, to excite the stimulable phosphor particles in the storage panel whereby releasing the radiation energy stored in the phosphor particles as light emission;
collecting the light emission from both surface sides of the radiation image storage panel;
converting the light emission collected on each side into electric signals;
combining the electric signals having been converted from the light emission collected on each side; and
reproducing the object in the form of a radiation image from the combined electric signals.
In the present specification, the xe2x80x9cmean particle sizexe2x80x9d means an average size of locally positioned phosphor particles.